The present invention has the background thereof in the need to shut off the air supply to an engine when, for instance, the gearing from the same is to be changed. Air intake ducts for internal combustion engines generally comprise a compressor, which is intended to provide a desired positive pressure upstream the engine when the same is active. The increased air pressure is aimed at in order to obtain a more expedient filling of the cylinders of the engine when the inlet valves of the cylinders are opened upon power output, i.e., normal operation. Thereby, the air intake duct accordingly has to co-operate with some form of valve in order to shut off the air supply when the accelerator pedal is disengaged as a consequence of continued, at least temporary, power output from the engine not being desired, for instance when the gearing from the engine is to be changed. In previously known combustion air supply arrangements, this valve has usually been located in one of two locations, either upstream the compressor or downstream the compressor. Furthermore, a plurality of variants and types of compressors are commercially available and the choice of compressor may affect the location of said valve. For instance, it may be mentioned that if a displacement compressor of screw-compressor type is used, the valve should absolutely be located upstream the same, since the screw compressor, as a consequence of the strength thereof, runs the risk of overloading other components unless the air supply to the compressor being restrained. In addition, however, a screw compressor runs harder when it operates in negative pressure, which entails a strong generation of heat in the system. For a compressor of, for instance, reciprocating-compressor type or centrifugal-compressor type, it does not generally matter if the valve is located upstream or downstream the same.
A troublesome disadvantage of placing the valve downstream the compressor is that, when the accelerator control is released, the compressor continues to suck in air and compress the same. The pressure increases and, furthermore, also the temperature of the air increases. The increasing air pressure downstream the compressor runs the risk of damaging other parts and details in the system, and in order to decrease the air pressure, a bypass line or duct is introduced into the system, which line extends from a position downstream to a position upstream the compressor, so that the air pressure downstream the compressor does not just continue to increase and increase. In spite of the air circulating through the compressor, through the bypass duct and once again through the compressor, the disadvantage remains that the pressure successively increases, although at a lower rate. This increased pressure, which is thrown out by a great force when the valve is opened, may be harmful to parts included in the engine.
In addition, yet a drawback arises when the valve is closed, more precisely in that the air is heated somewhat each time the same passes the compressor and accordingly is fed by energy. Supply of hot air into the cylinders of the engine when the valve once again is opened is not preferable, on the contrary, cold air is desired that has higher density, which gives an increased filling ratio and improved combustion of the fuel-air mixture in the cylinders of the engine.
In order to lower the temperature of the air, according to conventional methods, a charge-air cooler or intercooler is used, which is situated downstream the compressor and upstream the bypass duct. The charge-air cooler entails that the temperature of the air that has passed the compressor is lowered in the same before the air is recirculated through the bypass duct. In this way, a more advantageous temperature is obtained, but the charge-air cooler is strained to an unnecessary extent, whereupon its own temperature rises, which shortens the service life and which furthermore has a negative effect on the cooling capacity of the charge-air cooler when the valve is opened upon another step on the gas.
If the valve instead is located upstream the compressor, on one hand, no bypass duct is needed, but if the system, on the other hand, in addition comprises a charge-air cooler, other troublesome problems arise. When the valve closes as a consequence of the accelerator control being released, soon a negative pressure arises in the system between the valve and the engine, i.e., in the part of the system where the compressor and the charge-air cooler are located. When the valve is opened upon another step on the gas, the desired direct response from the engine does not occur, since first the pressure has to be built up in the entire system before the desired effect is obtained. That is, the system in general and the charge-air cooler in particular act as a lagging bellows, which entails that as small a charge-air cooler as possible is demanded, but with the disadvantage that with decreasing size of the charge-air cooler, also the cooling capacity decreases in normal operation.